Interpretive Summary: Western juniper has expanded into 8.5 million acres of rangeland in the western US, significantly impacting sagebrush-steppe ecosystems by suppressing understory vegetation. Estimating the extensive impacts of woody plant encroachment on rangelands is relatively difficult with most remote sensing technologies which only estimate 2-dimensional vegetation characteristics. In this study, we used Light Detection and Ranging (LiDAR) to characterize the three dimensional structure of juniper trees in order to estimate the degree of encroachment as well as the height, canopy volume and biomass. We found that LiDAR derived vegetation characteristics were highly correlated with ground measurements. This technology can, therefore, be effectively used by management agencies to characterizing current invasion status and trajectory of invasive woody plants, as a tool for prioritizing areas for landscape-scale management treatments, and to estimate the relative amount of carbon being stored in different plant communities across the landscape.

Technical Abstract:
Woody encroachment is a globally occurring phenomenon that is thought to contribute significantly to the global carbon (C) sink. The C contribution needs to be estimated at regional and local scales to address large uncertainties present in the global- and continental-scale estimates and guide regional policy and management in balancing restoration activities with greenhouse gas mitigation goals. We present a new technique to estimate aboveground C storage in juniper woodlands as a function of encroachment phase using high-resolution, 3-dimensional lidar measurements of individual juniper trees. A total of 2,613 juniper stems were mapped and measured in 85 field plots (30 x 30 m), where mean juniper stem density was 288 stems/ha. Mean mature tree height was 7.3 m with a mean age of 72 years. Lidar-derived juniper tree heights were strongly correlated with the field-measured individual tree heights (p < 0.001, Adj. R2 = 0.84). Lidar-derived juniper tree canopy cover and density (30 x 30 m cells) were also significantly correlated with field measurements (p = 0.04, Adj. R2 = 0.82 and p < 0.001, Adj. R2 = 0.79, respectively). Juniper tree canopy cover and density were predicted in 30 x 30 m cells using a regression approach. A total of 60,628 juniper tree crowns were identified across the watershed with segmentation analysis of the lidar point cloud data. Aboveground total biomass of individual trees were then estimated using a regression model with lidar-derived height and crown area as predictor variables (Adj. R2 = 0.77, p < 0.001). The mean aboveground woody biomass was 13.53 Mg/ha with a mean aboveground woody C storage of 677 g/m2. Later successional phases of woody encroachment had, on average, twice the aboveground C storage compared to the earliest phase. Woody encroachment might be more successfully managed and balanced with C storage goals by identifying priority areas in the earlier stages of encroachment where intensive treatments are most effective.